The present invention relates to a process for the manufacture of ferromagnetic metal particles, consisting essentially of iron, which are distinguished by a narrow particle size distribution coupled with a pronounced acicular shape, by reducing acicular iron oxides with gaseous reducing agents.
Because of their high saturation magnetization and the high coercive force achieved, ferromagnetic metal powders and thin metal layers are of particular interest for the manufacture of magnetic recording media. This is related to the fact that they enable the energy product and the information density to be substantially increased, so that, inter alia, narrower signal widths and higher signal amplitudes are achievable with such recording media. Thin metal layers have the further advantage over pigments that the ideal packing density of 100% can be achieved because no binder which is otherwise necessary is present. However, the cost of manufacture of the said metal layers is high, and in particular their use for magnetic recording tapes presents problems due to the mechanics of the recorder. At the optimum thickness of about 1 .mu.m or less, the surface of the layer must be very smooth because of head/tape contact, the slightest amount of abraded material or even dust being capable of causing destruction of the layer.
It is true that when using metal powders as magnetic pigments, the mechanical properties of the recording medium can be varied within wide limits by appropriate choice of the binder system, but the metal pigments must conform to special requirements in respect of shape, size and dispersibility.
Since a high coercive force and a high residual induction are essential prerequisites for magnetic pigments intended for magnetic coatings serving as data storage memories, the magnetic pigments used must exhibit single-domain behavior and furthermore the anisotropy already present or additionally achievable by magnetic orientation in the tape should only be slightly affected by external factors, eg. temperature or mechanical stresses, ie. the small particles should exhibit shape anisotropy and preferably be of acicular shape, and should in general have a size of from 10.sup.2 to 10.sup.4 A.
Numerous processes for the manufacture of magnetic metal particles are disclosed in the patent literature. For example, in the process of U.S. Pat. No. 2,974,104 magnetic iron particles are deposited by electroplating from an electrolyte solution onto a liquid mercury cathode. The particles must be subsequently separated from the mercury by an expensive method.
The reduction of, for example, iron salts with hydrides (J. Appl. Phys., 32, 184S, (1961)) and the vacuum vaporization of metals followed by deposition as whiskers (J. Appl. Phys., 34, 2905 (1963)) have also been disclosed, but are of no interest for industrial purposes. Further, it has been disclosed that metal powders of the above type can be manufactured by reducing finely divided acicular metal compounds, eg. oxides, with hydrogen, or some other gaseous reducing agent. The reduction must be carried out at above 350.degree. C. if it is to take place at a rate appropriate for industrial purposes. However, this is attended by the problem of sintering of the resulting metal particles. As a result, the shape of the particles no longer conforms to that required to give the desired magnetic properties. To lower the reduction temperature, it has already been proposed to catalyze the reduction by applying silver or silver compounds to the surface of finely divided iron oxide (German Laid-Open Application DOS No. 2,014,500). Modification of the iron oxide, which is to be reduced, with tin (German Published Application DAS No. 1,907,691), with cobalt/nickel (German Published Application DAS No. 2,212,934) and with germanium, tin or aluminum (German Published Application DOS No. 1,902,270) is alleged to be similarly effective. However, if the reduction of the acicular starting compounds is catalyzed by the above metals, the resulting needles are in general far smaller than the starting product, and furthermore their length-to-width ratio is low. As a result, the end product exhibits a rather broad particle size spectrum and consequently a broad distribution of shape anisotropy. However, the literature discloses that the dependence of the coercive force and residual induction of magnetic materials on their particle size is very great when the particles are of the order of size of single-domain particles (Kneller, Ferromagnetismus, Springer-Verlag 1962, 437 et seq.). If to this are added the effects resulting from the presence of a proportion of superparamagnetic particles which may be formed as fragments when using the above method, then such magnetic pigments are highly unsuitable-for example because of their poor maximum output level-for use in the manufacture of magnetic recording media. With such heterogeneous mixtures, the magnetic field strength required to reverse the magnetization of the particles varies greatly, and the distribution of the residual magnetization as a function of the applied external field also gives a curve of low slope.
It is an object of the present invention to provide a method of producing acicular ferromagnetic metal particles which are distinguished by a narrow particle size spectrum coupled with a pronounced acicular shape and which therefore exhibit a narrow field strength distribution, a very steep residual induction curve and only slight temperature dependence of the magnetic properties.